Stereoscopic video display systems that display a field-sequential image have been described in U.S. Pat. Nos. 4,523,226, issued June 11, 1985 to Lipton, and in 4,562,463, issued Dec. 31, 1985 to Lipton.
In one version of this type of display system (described in U.S. Pat. No. 4,562,463 with reference to FIGS. 1-3) an observer views a display screen through powered electro-optical shutters (elements 15 and 16 of U.S. Pat. No. 4,562,463) which are synchronized with the field-sequential image at the field rate. However, use of such active occluding shutters has a number of drawbacks. The active shutters must be synchronized with the field-sequential display by cable or wireless transmission means so the shutters will open and close at field rate. Since each shutter is open only half of the time, when viewing the environment surrounding the display, such as printed material, the ambient illumination is reduced by the duty cycle, i.e., by a factor of two. In addition, in the transmissive state, conventional active electro-optical shutters impose the attenuation of two sheet linear polarizers with parallel axes in front of each eye. If another video display is used, a disturbing "roll bar" will be seen, since the shutters may not be synchronized to the field rate of another display.
On the other hand, the use of what we call "onscreen modulation" systems has also been proposed. Such systems employ a large electro-optical polarization switching device, which covers the display screen and alters the polarization characteristic of the transmitted light at field rate, with passive selection devices including sheet polarizers that have no intrinsic duty cycle. In this type of onscreen modulation system, the brightness of the environment surrounding the display is reduced only by the attenuation of a single polarizing sheet. Moreover, there will be no "roll bar" artifact when looking at other, unsynchronized video displays.
U.S. Pat. Nos. 3,858,001, issued Dec. 31, 1974 to Bonne; 4,281,341, issued July 28, 1981 to Byatt; Japanese Patent Application (Kokai) No. 52-110516 by Fujita; and above-mentioned U.S. Pat. No. 4,562,463 (with reference to FIG. 4 thereof) have suggested the use of onscreen modulation techniques employing a large liquid crystal cell which alters the characteristic of polarized light at field rate, in which the observer views the display screen through passive eyeglasses with sheet linear polarizer filters. Together the modulator and sheet polarizers in the glasses form a shutter for image selection for a field sequential stereoscopic video display.
We can also find an early reference to a related concept by Reynolds, who, in U.S. Pat. No. 2,417,446, issued Mar. 19, 1947, suggests using a Kerr Cell for a variable retarder. In Reynolds' concept, a sheet linear polarizer is employed at the CRT (cathode ray tube) screen with active elements (Kerr Cells) and passive sheet linear polarizers at each eye.
Conventional video display systems employing onscreen modulation typically include a video screen covered by a sheet linear polarizer and an electrooptical variable halfwave retarder cell. Each observer wears linear polarizing spectacles to view light transmitted from the screen through the linear polarizer and halfwave retarder cell. The halfwave cell is typically liquid crystal (LC) cell that is switched from an isotropic state (at high potential) to a birefringent state (at low potential) at field rate. If the axis of the onscreen linear polarizer is at 45 degrees to the optical axis of the variable halfwave retarder, then the plane of polarized light exiting the retarder and visible to the observer will have its axis alternating between orthogonal states with each successive field. Hence, linear polarizers mounted with orthogonal axes for the left and right lenses in the spectacles alternately occlude or transmit the appropriate image.
Such conventional systems thus employ a liquid crystal suutter including first and second linear polarizers whose axes are orthogonal, with a liquid crystal cell interposed between the polarizers, and with the axis of the cell bisecting the polarizer axes. In the shutter's transmissive state, low voltage is applied to the cell, so that the cell is a uniaxial birefringent crystal which resolves the incident wave into two orthogonal component waves of linear polarized light, polarized parallel and perpendicular to the principal axis, respectively. The rate of propagation of light through the crystal is different for the two component waves. In passing through the halfwave retarder, the fast wave is retarded 180.degree. less than the slow wave is retarded. The vector sum of the emerging fast and slow electric vectors results in a reflection about the principal axis of the initial polarization vector. If the initial polarization angle was 45 degrees, the reflection is equivalent to a rotation of the polarization angle by 90 degrees.
In the shutter's occluded state, characterized by high voltage, the electric vectors of an incoming wave are not rotated, and the liquid crystal cell is in an isotropic state. In this state, the index of refraction of the liquid crystal material is the same in every direction, and there is no retardation effect.
However, conventional onscreen modulation systems employing passive sheet linear polarizers to view transmitted light from a video screen have the following disadvantage. Because of the law of Malus (described, for example, in Fundamental of Optics, Fourth Edition, Jenkins and White, McGraw-Hill, 1976), head-tipping of only a few degrees by the viewer will lead to an unacceptable increase in image crosstalk. The law of Malus relates the intensity I of linear polarized light transmitted by a linear analyzer, to the intensity I.sub.o of the incident linear polarized light, and the angle b between the plane of the axis of incident polarized light and the plane of the axis of the analyzer, by the expression: EQU I=I.sub.o cos .sup.2 b.
Thus, a small change in the angle b will result in a large change in transmission. Accordingly, only a little head tipping leads to the perception of the unwanted images by the eyes when viewing through linear polarizing spectacles.
In spite of the greatly improved environmental image brightness, the convenience, and the lighter weight of the passive spectacles compared with active shuttering goggles, for many applications it may be unacceptable to require that the viewer's head remain rather rigidly in place while observing the image.
If circular, rather than linear, polarized light could be employed, then polarizer extinction would not be angularly dependent, and head-tipping would not produce ghosting, as suggested by Land in U.S. Pat. No. 2,099,694, issued Nov. 23, 1937 (at page 2, left column, lines 62-68). In the case of circularly polarized light, the Law of Malus does not apply. Given a source of incident circularly polarized light of one handedness, and a circular polarizer analyzer of the opposite handedness, the transmitted intensity of light remains substantially constant with rotation of the analyzer with respect to the incident light. Accordingly, the unwanted field remains occluded from view.
However, conventional large liquid crystal cell (LLCC) devices are unsuitable for producing circular polarized light of the type suitable for use in a stereoscopic video display system.
For example, we believe that it is disadvantageous to employ a conventional LLCC in conjunction with a linear polarizer as described above, in combination with a conventional quarter-wave retarder as suggested in "Compatible 3-D Television: the State of the Art," by Balasubramonian, et al. in SPIE, Volume 402, 1983 (pp. 100-106). FIG. 1 illustrates this conventional approach. Video monitor 1 is fed a video signal from video source 5 via cable 6. CRT (or similar display screen) 2 is viewed by an observer through a linear polarizer 3, and LLCC half-wave retarder 4 (also referred to as LLCC 4). LLCC 4 is powered by controller 8 via cable 9. Controller 8 senses vertical synchronization pulses of video source 5 via cable 7 and uses these sync pulses to trigger LLCC 4, which varies optically from the isotropic to birefringent condition at video field rate. Quarter-wave retarder 13 is placed in front of LLLC 4. The absorption axis of polarizer 3 is oriented at 45 degrees to the rub axis of LLCC 4, and the fast optical axis of retarder 13 must be parallel to the rub axis of LLCC 4. Linear polarizer 3, LLCC 4, and quarter-wave retarder 13 are in intimate juxtaposition and mounted in front of CRT screen 2. Analyzing spectacles 10 with circular polarizing filters 11 and 12 are used for viewing the image. An example of a commercially available LLCC of the type that may be use in the FIG. 1 system is the "pi-cell" having 12 inch diagonal, manufactured by Tektronix, Inc. The disadvantages of the FIG. 1 arrangement will be discussed below.
In FIG. 2, we show a similar disadvantageous conventional approach for producing circular polarized light in a stereoscopic video display system. All elements of the FIG. 2 system are the same as those in FIG. 1, except that quarter-wave retarder 13 has been removed, and linear polarizer 3 has been replaced by conventional circular polarizer 14. The axis of the linear polarizer portion of circular polarizer 14 is oriented at 45 degrees to the rub axis of LLCC 4.
The observer views screen 2 through circular polarizer 14 and LLCC 4, which are in intimate juxtaposition and mounted at the screen, using glasses 10, which have circular polarizer analyzers of opposite handedness 11 and 12. Elements 1, 2, and 5 may be replaced by a suitable motion picture projector, and LLCC 4 driven at the appropriate motion picture field rate (determined by the projector speed and the length of each field segment on the film), in a conventional variation on the FIG. 2 system.
Circular polarizers commercially available from Polaroid Corporation (having product designation HN37CP), or similar circular polarizers, were used for projection of stereoscopic films in a few motion picture theaters in 1983. A motion picture projection system using such a circular polarizer is described in Three-Dimensional Projection with Circular Polarizer, by Walworth, et al., SPIE, Vol.462, Optics in Entertainment II (1984), pp.64-68.
The Polaroid circular polarizer consists of a sheet linear polarizer and a quarter-wave retarder bonded together with 45 degrees between their axes. In the FIG. 2 system, the sheet linear polarizer side of circular polarizer 14 faces CRT 2, and the quarter-wave retarder side faces LLCC 4. Light from the phosphor of CRT 2 passes through circular polarizer 14, and then through LLCC 4. The light output is circular polarized light, alternately left-handed and right-handed, as LLCC 4 is switched at the field rate.
Our experiments have shown that commercially available LLCC's, though available with diagonals of 12 inches or more, do not have adequate performance with respect to dynamic range and decay time (the time in which the shutter changes from its transmissive to its occluded state for a stereoscopic selection device application. The dynamic range of a shutter is defined as the ratio of its transmission in its on state to its transmission in its off state. For a 12 inch pi-cell manufactured by Tektronix used with the FIG. 1 and FIG. 2 systems described above, we have measured a dynamic range for one eye about fifty percent greater than for the other eye, that is 12:1 for one eye, and 8:1 for the other. Moreover, for either eye, the dynamic range of either of these conventional systems is unacceptably low. An equal dynamic range for each eye of many times the figures given above, is necessary for acceptable results. The result of the measured low add asymmetric dynamic range is an observed doubling or "ghosting" of the displayed image produced by either conventional system, because inadequate occlusion of the unwanted image leads to crosstalk.
In addition, althoug the present surface mode LLCC's (such as the Tektronix 12 inch diagonal pi-cell) have a fairly rapid rise time (the time in which the shutter changes from it occluded to its transmissive state), they have a slow decay time. The rise time and decay time are less than 1 millisecond and about 2 milliseconds, respectively. This asymmetry presents pooblems for a stereoscopic display since a portion of one set of fields (either the right or left) may show partial occlusion or discoloration as a result. The asymmetrical natures of the dynamic range and rise and decay times are closely related, and inherent in the construction of the conventional LLCC's. Since the vertical blanking interval of a raster scan video or computer graphics display is on the order of one millisecond, considerable improvement in speed is needed.
The asymmetrical nature of the dynamic range for the left and right eye arises from the fact that in the case of one eye, the analyzer (spectacle) axis must be perpendicular to the linear polarizer axis at the modulator, and the other eye must see through an analyzer with an axis oriented parallel with respect to the modulator polarizer linear axis. For the perpendicular case, the dynamic range is higher than for the parallel case.
In order for an on-screen switching device to produce an acceptable stereoscopic display, rise and decay time must be substantially the same and within the vertical blanking interval for a raster display. (Even faster rise and decay times are demanded by vector displays.) Moreover, the dynamic range must be substantially the same for both eyes, and the dynamic range must be many times greater than presently available from commercial LLCC's. Until the present invention, it was not known how to achieve these desired characteristics.